MARINE ISOPODA OF NEW ENGLAND, ETC. 409 



(third and fourth free segments) are indistinctly separated in the adult 

 females. 



The pleon is much alike in both sexes and the young, and consists of 

 six distinct segments, each of which bears a pair of appendages. The 

 first five pairs of these aj)pendages, or pleopods, are carried beneath the 

 pleon and subserve the purposes of respiration, while they are also used 

 in swimming. They consist of a short basal segment supporting two 

 rami, ciliated at the tip in the young. The uropods are directed back- 

 ward and are of fii-mer texture than the pleopods. They are ciliated 

 near the tip. 



Only a single species has yet been recognized within our limits, and 

 the male, female, and young will be described under the specific name. 



The striking sexual differences in this family have caused much con- 

 fusion, the males having been referred to one genus (Anceus)^ and the 

 females to another {Franiza), and even these genera have been referred 

 to different tribes or subfamilies. The true relationship of these forms, 

 long ago suspected by Leach, was first made known by M; Hesse,* who, 

 however, seems not to have stated it very clearly and perhaps did not 

 correctly apprehend it at first. His descriptions, however, of the 

 females of Anceus aj^ply to what had previously been regarded as the 

 female of Praniza, although he says in the same paper that Franiza is 

 only the larval state of Anceiis^ which is true only of the young, or larval 

 forms, or the then supposed males of Franiza. This fomily has been 

 further investigated by Bate, Westwood, and Dohrn, to whose writings 

 the reader is referred. It may be here remarked that Bate and West- 

 wood in their account of the structure of Anceus^ in the second volume 

 of the British Sessile-Eyed Crustacea, appear to have overlooked the last 

 thoracic segment, and suppose that either the first or second segment 

 must be wanting. Dohrn calls attention to the rudimentary (or embry- 

 onic) condition of the seventh thoracic segment as the one missing to 

 complete the normal number, but describes and figures! as "untere" 

 and "obere Mundextremitiit" ("verwandeltes erstes" and '^zweites 

 Gnathopoden Paar"j what I regard as the maxillipeds and first pair of 

 thoracic legs, or, according to Spence Bate's terminology, which Dohrn 

 seems to have misapprehended, the maxillii^eds and the first iDair of 

 gnathopods. The second pair of gnathopods are pediform as usual in 

 the Isopoda, and are the first of the five i:)airs of legs. Of the five 

 pairs of pereiopods normally present, only four are developed in the 

 Gnathiidae. The family is thus remarkable in the order both for the 

 transformations undergone in its development, and for the retention 

 after all of an embryonic feature. 



Having discarded the names Anceus and Franiza for reasons given 

 below, I have also rejected the family name Anceidoi and substituted 

 for it a name, suggested by Bate and Westwood and derived from that 



* Aun. Sci. nat. , IV, torn, is, p. 106, 1858. 

 tZeit. Wis8. Zool., xx, taf. vii, figures 24 and 25. 



